[SPR-11317] ServletWebRequest.checkNotModified() methods should treat HEAD identically to GET

[malkusch]

SPR-11317 ServletWebRequest.checkNotModified() methods don't treat a HEAD request identically to a GET request (in fact they don't consider HEAD at all).

HTTP says:

The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request SHOULD be identical
to the information sent in response to a GET request.

Spring's reference documentation suggests to call WebRequest.checkNotModified() to set the status code to 304:

calling request.checkNotModified(lastModified) and returning null. The
former sets the response status to 304

As a consequence of the current implementation, applications which follow that recommendation violate against HTTP. In simple words: I had some fun debugging my application for a missing 304 status code while firing HEAD requests with wget --server-response --spider on it.

I suggest to respect HEAD in the mentioned methods identically to GET.

Test cases

@Test
public void checkNotModifiedTimeStampForHEAD() {
    long currentTime = new Date().getTime();
    servletRequest.setMethod("HEAD");
    servletRequest.addHeader("If-Modified-Since", currentTime);

    request.checkNotModified(currentTime);

    assertEquals(304, servletResponse.getStatus());
}

@Test
public void checkNotModifiedETagForHEAD() {
    String eTag = "\"Foo\"";
    servletRequest.setMethod("HEAD");
    servletRequest.addHeader("If-None-Match", eTag );

    request.checkNotModified(eTag);

    assertEquals(304, servletResponse.getStatus());
}

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